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The nucleation and growth of cavitation bubbles few micrometers in size in water generated by a 60 ps 515 nm fiber laser is observed and visualized near nucleation threshold. The study is performed by monitoring the plasma size, the cavitation bubble size and the emitted shock waves. The latter two aspects are supported by the Gilmore model using a Noble-Abel-stiffened-gas (NASG) equations of state. For the first time, two types of cavitation events are identified and visualized that exhibit a difference of more than two orders of magnitude in the excitation energy converted to mechanical effects with minimal change in excitation laser pulse energy. The result is localized cavitation and reduced mechanical stress on water-based media with potentially positive implications for laser treatments of biological tissue.

• The Influences of coupling errors on laser transmission were explored with the established geometric and simulation model. • The thresholds of the laser-induced breakdown were measured. The laser transmission efficiency could be larger than 81.2 %. • A Gaussian shape distribution of laser in electrode exit, and the laser processing capability in large depth were verified. Laser and electrochemical machining (LECM) has been increasingly adopted to improve the efficiency and surface quality of micromachining. The coupling and high-efficiency transmission of laser in electrolyte are critical to the implementation of LECM, which were rarely studied in previous research. In this study, the geometric and simulation model of laser coupling and transmission within the electrolyte jet has been established for laser and shaped tube electrochemical machining (Laser-STEM). The critical coupling condition and transmission efficiency of laser beam was investigated theoretically. Influences of laser coupling errors on the laser transmission efficiency and laser power intensity distribution at the exit of hybrid tool electrode were explored. Results indicated that the angular offset had the greatest impact on laser transmission efficiency, while the longitudinal offset impact the least, which were accordance with the experimental results. The thresholds of laser power intensity and the frequency of the laser-induced electrolyte breakdown were measured. The thresholds for laser-induced breakdown in NaNO 3 solution were higher than that in NaCl solution. The laser power intensity threshold of laser-induced electrolyte breakdown increased with the increase of focal length, and the frequency of smaller than 4.5 could be obtained with the laser power density ranging from 2.35e9 to 1.06e10 W/cm2 at the focal spot. The distribution trended to more uniform at the exit with the increase of hybrid tool electrode length. A laser coupling efficiency of larger than 81.2 % has been achieved by utilizing the hybrid tool electrode with NA of 0.534 and acceptance angle of 32.3°. Finally, experiments with a designed raster-shaped workpiece were conducted to verify the feasibility of laser transmission in large depth in the Laser-STEM process. The high aspect-ratio small holes with a depth of 50 mm, and a diameter of 1.3 mm have been processed by using Laser-STEM. [ABSTRACT FROM AUTHOR]

A wide-field high resolution microscopy with sub-micron resolution and high contrast image was demonstrated by using the isotropy frequency-domain extension imaging (IFEI), which was successfully applied to submicron-scale structure detection of fused silica wafer for the first time. Employing the frequency-domain extension and the isotropic period matching, the IFEI extended the bandwidth of coherent transfer function of the conventional wide-field microscopy, thereby improving the resolution to break the diffraction limit. In this method, a quantitative measurement of 200 nm was achieved using fused silica wafer with prefabricated etched structure on the surface. By imaging 81 nm gold nanoparticles, the system resolution calibration was achieved. And corresponding system’s lateral resolution of approximately 179.5 nm was obtained, which was 1.72 times higher than the conventional wide-field microscopy image. Moreover, the reconstructed IFEI images of ultra-precision processed fused silica optics were demonstrated to present the finer structures of sub-micron digs and scratches formed in the optics manufacturing process.

The aim of this study is to evaluate the capability of femtosecond Laser Induced Breakdown Spectroscopy (fsLIBS) to discriminate between normal and cancerous bone, with implications to femtosecond laser surgery procedures. The main advantage of using femtosecond lasers for surgery is that the same laser that is being used to ablate can also be used for a feedback system to prevent ablation of certain tissues. For bone tumor removal, this technique has the potential to reduce the number of repeat surgeries that currently must be performed due to incomplete removal of the tumor mass. In this paper, we performed fsLIBS on primary bone tumor, secondary tumor in bone, and normal bone. These tissues were excised from consenting patients and processed through the UC Davis Cancer Center Biorepository. For comparison, each tumor sample had a matched normal bone sample. fsLIBS was performed to characterize the spectral signatures of each tissue type. A minimum of 20 spectra were acquired for each sample. We did not detect significant differences between the fsLIBS spectra of secondary bone tumors and their matched normal bone samples, likely due to the heterogeneous nature of secondary bone tumors, with normal and cancerous tissue intermingling. However, we did observe an increase in the fsLIBS magnesium peak intensity relative to the calcium peak intensity for the primary bone tumor samples compared to the normal bone samples. These results show the potential of using femtosecond lasers for both ablation and a real-time feedback control system for treatment of primary bone tumors.

Studies of the opto-acoustic effects accompanying laser breakdown in water generated by focused laser and ultrasonic radiation are performed. For the first time, the experimental results were obtained showing sharply increasing effects of acoustic emission from the breakdown zone under the combined influence of laser and ultrasonic irradiation. Experiments were performed using the nanosecond pulses of a Nd:YAG laser operating at a wavelength of 532 nm. Acoustic radiation was generated by acoustic focusing systems at various frequencies. It is shown that recording of acoustic emission from the breakdown zone makes it possible to study the thresholds and dynamics of the laser breakdown, which coincide with the high-speed optical methods. The results obtained make it possible to find applications of the acoustic method of diagnostics of laser breakdown and cavitation in opaque environments for which optical methods are not applicable. The effect of the increase in the resolution of the spectral lines of chemical elements in seawater under the action of an ultrasonic field is established. This effect indicates the possibility of development of a combined method of laser–spark spectroscopy using ultrasound, which will make it possible to approach the sensitivity of the precision chemical analysis, and which should be more practical and advanced because it yields a high efficiency and detailed measurements. [ABSTRACT FROM AUTHOR]

This work investigates the influence of pulse energy on machining characteristics using a 10 ps laser during Laser induced plasma micromachining (LIPMM) process. An axisymmetric model combining the effects of cascade, multiphoton ionization, and recombination and diffusion losses was developed to simulate the spatial and temporal plasma profiles at various pulse energies in distilled water. Thereafter, micro-channels with the corresponding pulse energies were created on stainless steel using LIPMM, and the plasma focusing process, depth and width of the machined channels were investigated. It was found that pulse energy had a negligible effect on the variation of the focusing distance during the LIPMM focusing process. The simulations and experimental geometric features showed similar qualitative trends with the increasing pulse energy at the plasma’s focal plane. However, the pulse energy had a significant influence on the machined depth, but a much lesser influence on the machined width. [ABSTRACT FROM AUTHOR]

The experimental parameters for observing the characteristic of laser induced breakdown (LIB) plasma are optimized in the atmospheric environment. It shows that when the laser pulse energy is 21 mJ, the focal length of lens is 15 cm and the sample is 2 mm in front of the lens focus, the spectral intensity and the ratio of signal to noise (SBR) of LIB plasma are highest. It is helpful to improve the detection accuracy of the technique of laser induced breakdown spectroscopy (LIBS). The LIB spectra of a special soil samples are observed with the optimized parameters. The elements Fe, Mn, Ca, W, Co, Ti, Si, Cr and V contained in the soil sample are determined based on the spectral peaks. The electron temperature in plasma and the quantities of Mn, Ca in the soil are obtained further. [ABSTRACT FROM AUTHOR]

In this paper, laser induced optical breakdown in colloidal nanoparticles was studied by using pump- probe beam technique. Colloidal nanoparticles of Ag (as a good conductor), Al 2 O 3 and TiO 2 (with good dielectric properties) were used in this investigation. The optical breakdown was induced by an Nd:YAG laser beam (operating at 1064 nm with pulse duration ∼30 ns). A small portion of the beam was taken by an optical splitter and used as probe beam. The time varying transmission of the probe beam transversely through the plasma was measured during the breakdown process. According to the results, the nanoparticles characteristics and pump beam intensity have significant influence in the breakdown process. Our results also show dissimilar dynamic behaviors for conductor and dielectric nanoparticles at different pump intensity. The results are useful for nanoparticle synthesis by laser ablation in distilled water in which the optical breakdown intensity threshold of ambient water influenced by generated nanoparticles. [ABSTRACT FROM AUTHOR]

Abstract: This paper presents a numerical investigation of the measurements that were carried out by Yagi et al. to study the breakdown of molecular hydrogen induced by short laser of wavelength 248nm and pulse duration 20ns. The aim of the study is to give a detailed description of the physical processes which contributed to the breakdown of molecular hydrogen at focused intensities between 4×1012 W/cm2 and 8.0×1012 W/cm2 over gas pressure range extended from 150 to 7000Torr. The applied computer simulation model is based on the numerical solution of the time dependent Boltzmann equation and a set of rate equations that describe the rate of change of the formed excited states population. The experimentally measured rate constants and cross-sections for the various physical processes involved in the model are used in the calculations. Provision is made for the electron impact ionization and photoionization of the excited states. The former process is incorporated parametrically in the calculation owing to the lack of quantitative description of this process. Computations are performed at each gas pressure. The calculated threshold intensities are found to be in good agreement with the experimentally measured ones, both showing a noticeable increase in the higher pressure region. Moreover, calculation of the electron energy distribution function (EEDF) and its parameters demonstrated the exact correlation between gas pressure and the physical processes responsible for determining the breakdown threshold intensity. [Copyright &y& Elsevier]

Abstract: Laser induced breakdown in water represents an abrupt high intensity photoacoustic phenomenon. The pressure front that appears at its site represents an ultrasonic shock wave spreading into surrounding water. At the same time the cavitation bubble develops. The effect of both is particularly important in ocular microsurgery, where Q-switched lasers are used to vaporize the tissue in procedures such as posterior capsulotomy. Formation of the cavitation bubble and the propagation of the sound wave were analyzed by an optodynamic experimental set-up. It is based on two-dimensional scanning technique using a beam deflection probe. From the beam deflection probe signals the time of flights for both the sound wave and the cavitation bubble were determined. By using Hugoniot curves and the two-dimensional time of flight data field the pressure amplitude in the front was estimated. Simultaneously the expansion of the cavitation bubble was also analyzed and its energy was determined. [Copyright &y& Elsevier]

The therapeutic application of laser light is required to minimize defects in the non-irradiated tissue. The primary mechanism of interaction is determined by the duration of laser action. In the case of continuous wave laser light a tissue layer surrounding the irradiated volume is thermally affected. With the pulses of a Q-switched laser (duration some ns) tissue cutting will be obtained by the laser-induced breakdown. To be able to distinguish between thermal and mechanical effects by histological examination, experiments were performed with laser pulse durations of 8ns and 100μs under the same conditions with a Nd-YAG laser at 1064nm. The beam was focused through air below the tissue surface. The beam geometry in the focal region was identical for both cases. The defective region after irradiation could be divided into four zones surrounding a crater. In the μs-experiments the zones corresponded to the temperature distribution in the tissue, so the changes were all classified as thermal. In the ns-experiments, in general larger craters were found. Increasing the number of pulses to 200 the picture is similar to that produced with μs-pulses. These results show that a few ns-pulses suffice to form a crater. Additional ns-pulses lead to heat accumulation and produce thermal lesions like those of the μs-case. [ABSTRACT FROM AUTHOR]